esp8266/modnetwork: Introduce interface .config() method.

Allows to set (in case keyword args are given) or query (in case a single
"symbolic keyword" (a string, value is the same as keyword)) arbitrary
interface paramters (i.e. extensible and adaptable to various hardware).

Example usage:

ap_if = network.WLAN(1)
ap_if.config(essid="MicroPython on Air")
print(ap_if.config("essid"))